A person, the vast majority of the time a female, who acts too nice to be real in order to lure in pathetic dopes and use/betray them, frequently crushing the victim's soul in the process. Symptoms of fakeness include wearing a huge, phony, Botox-looking smile the entire time, saying "Hiiiiii! How are you!" a lot, using an overly sweet tone of voice that's so dripping with sap it makes you want to vomit, and just generally being overly perky all the time. A fake chooses her victims very carefully, usually very naive individuals, waits until the victim has divulged their deepest, darkest secrets, then spreads them all over the school and makes sure the victim is absolutely miserable. AVOID AT ALL COSTS.

Fake: Hiiiiiii! You look really lonely. Do you need a friend? You can trust me with anything.
Victim: That would be really nice...I never have had any real friends before.
Fake (under breath): Ha! She took the bait! THIS is gonna be good!

Someone who acts fake is someone who talks shit about a certain person, and then when that certain person is around they are nice to them

Becky- I love how last night Tiffany And Devonann were talking shit about Rachel, and now they are over their hugging her and laughing with her.
Stevie- Yeha poor Rachel doesn't know that those bitches are totally fake to her